Efficient light delivery for photodynamic therapy
The presence of metastasis is one of the most important prognosis factors of cancer [1]. While understanding factors governing metastasis is important, it is also critical to eliminate cancer cells sufficiently early preventing metastasis while sparing surrounding normal tissues. Photodynamic therapy (PDT) has been shown to be a powerful method in which a light-activatable molecule that can selectively accumulate in tumors is used to generate local cytotoxicity [2].

Tissue is a heterogeneous, turbid medium, which causes multiple scattering of a light propagating within it; thus the light becomes diffused over transmission distances of a few mean-free paths. For typical PDT-relevant wavelengths, one mean-free path is on the order of 100 µm, thus, the depth at which a photosensitizer can be activated efficiently is of the order of a few millimeters. Moreover, the ability to focus the illumination beam to a target location within a lesion is lost. However, elastic-scattering effects are deterministic and reversible [3]. The goal of this study is to develop efficient methods for appropriate illumination wavefield so that the light both penetrates deep within the sample, and is concentrated at a region of interest. In the context of PDT, this would permit lower light doses to be delivered while achieving the same drug activation effect, reducing the risk of tissue photodamage, and increasing the depth at which treatment may be effective, potentially up to several centimeters.

The application of turbidity-suppression for enhanced PDT is highly clinically relevant. PDT is presently in clinical trials for both ovarian [4-8] and pancreatic [9, 10] cancers. Pancreatic cancer patients are often moribund and poor candidates for therapies with high systemic toxicity, making PDT an attractive option, particularly with the greater control in light delivery that the new turbidity surpression approach will offer.